How would you explain Thoreau's reasons for leaving Walden Pond?

The
other answers rightly quote Thoreaus own stated reason for leaving the woods: because he had
begun to fall into a rut in his forest existence, and he was no longer far from the beaten path
but treading it daily. He believed it was time to move on because he had other lives to live and
had spent quite enough time and energy on that one.

Earlier in chapter 18,
Thoreau addresses the question of travel for its own sake and suggests that this is pointless
unless one is able to learn something new. Having left the woods, he feels that his experiment
taught him an important lesson: that if a man strives for his dreams, this will teach him a new
way of seeing the world, and new paths will open themselves up to him.

In
pursuit of his dreams of learning, then, Thoreau must move on because the woods have nothing
else to reveal to him, but also he is now convinced of the value of going where dreams compel
him to go. Thoreaus quest is not for love or money, but for truth, and he...